1. Field of the Invention
The invention relates to a method for preparing substituted diesters of pyridine-2,3-dicarboxylic acids by reacting 2,3-dichloropyridines with carbon monoxide and an alcohol in the presence of a catalyst and a base. The diesters which can be prepared according to the invention have the general formula: wherein:                R represents C1-6-alkyl, C3-6-cycloakyl, aryl or arylalkyl, and        R1 to R3, independently of one another, represent hydrogen, C1-6-alkyl, fluorinated C1-6-alkyl, C1-6-alkoxy, (C1-6-alkoxy)-C1-6-alkyl or (C1-6-alkoxy)carbonyl.        
2. Background Art
Numerous compounds having this structure are important herbicides or synthesis building blocks for the preparation of herbicides (U.S. Pat. No. 5,288,866). The synthesis of these known compounds usually starts from the corresponding carboxylic acids or carboxylic acid derivatives (acid chlorides, nitrites). These, however, are often not readily accessible and, therefore, are expensive.
A further method (U.S. Pat. No. 5,296,601) for preparing diethyl pyridine-2,3-dicarboxylates starts from 2,3-dichloropyridine which is reacted with ethanol and carbon monoxide in the presence of a palladium complex, which contains 1,4-bis(diphenylphosphino)butane as ligands, and sodium carbonate as the base to give the diester. However, the yield is so low (<3 percent) that an industrial application of the method would be entirely uneconomical.